1. Field of the Invention
This invention relates to an optical system and an image display apparatus using the same, and is suitable for a head mounted display or the like adapted to enlarge and cause image information displayed on display means such as a liquid crystal display element to be observed through an optical element having an appropriately set free curved surface. Also it relates to an optical system suitable for an image pickup (imaging) apparatus enhanced in the degree of freedom of layout by the use of optical elements which has free curved surfaces.
2. Related Background Art
A head mount type image displaying apparatus (head mounted display) using a display element such as a CRT or an LCD and adapted to enlarge and cause an image displayed on this display element to be observed through an optical system is well known.
FIG. 7 of the accompanying drawings is a schematic view of the essential portions of an image displaying apparatus using a coaxial concave mirror according to the prior art. In FIG. 7, a beam from an image displayed on a display element 61 is reflected by a half mirror 62 and is caused to be incident on a concave mirror 63. The beam reflected by the concave mirror 63 is directed to an observer E through the half mirror 62. The image displayed on the display element 61 is formed as a virtual image enlarged by the concave mirror 63. Thereby, the observer observes the enlarged virtual image of the image displayed on the display element 61.
Also, for example, in Japanese Patent Application Laid-Open No. 7-333551, Japanese Patent Application Laid-Open No. 8-50256, Japanese Patent Application Laid-Open No. 8-160340 and Japanese Patent Application Laid-Open No. 8-179238, there are proposed image displaying apparatuses each of which uses an LCD (liquid crystal display) as display means for displaying an image and a thin type prism as an observation optical system and has generally been made thin.
FIG. 8 of the accompanying drawings is a schematic view of the essential portions of an image displaying apparatus proposed in Japanese Patent Application Laid-Open No. 7-333551. In FIG. 8, a light emitted from an LCD 51 is caused to be incident on the incidence surface 53 of a compact prism 52. The beam is folded between a total reflection surface 54 having a curvature formed on the compact prism 52 and a reflecting surface 55, whereafter the beam is caused to emerge from the compact prism 52 through the surface 54 and is directed to an observer E. Thereby, the virtual image of an image displayed on the display means (LCD) 51 is formed and the observer E may observe this virtual image. The reflecting surface 55 of the compact prism 52 comprises an eccentric free curved surface comprised of an eccentric non-rotation-symmetrical surface (a surface differing in optical power depending on the azimuth angle, i.e., a so-called free curved surface or free-form surface).
The type of the optical system shown in FIG. 8, as compared with the type using the prior-art coaxial concave mirror shown in FIG. 7, has the feature that the thinning of the entire apparatus and the widening of the angle of observation field are easy.
Heretofore, the image displaying apparatuses such as the head mounted display and a spectacle type display have been required to be generally downsized and made lighter in weight because these apparatuses are mounted on the head. Particularly, when the weight balance, appearances, etc. of the apparatus are taken into consideration, it is preferable that the apparatus be thin in the direction of observer's visual axis. Also, it is an important task to widen the angle of observation field to give power to the observation of the image displayed on the display means.
In recent years, the higher definition of an LCD or the like which is display means for displaying an image has been advanced, and there has been developed an LCD which has substantially the same number of pixels as in the prior art and is more compact than the prior-art article. Again in a case where such compact display means is used, it is desired to achieve an angle of field similar to that in the prior art.
In Japanese Patent Application Laid-Open No. 10-153748, an eccentric prism optical system and a relay lens system having refractive power are combined together, and image information displayed on display means is once formed as an intermediate image by the relay lens system, and then the image displayed on the display means is directed to an observer, whereby the wider angle of field of the optical system is achieved relative to the panel size.
As further improvements in the optical performance of this publication, optical systems of a type in which the internal reflecting surfaces of the eccentric prism are increased and the intermediate image of the image information displayed on the display means is formed in the eccentric prism and the intermediate image is directed to the observer, a type in which a second eccentric prism is provided to form an intermediate image, and a first eccentric prism leads the intermediate image to an observer, etc. are disclosed in Japanese Patent Application Laid-Open No. 2000-066106, Japanese Patent Application Laid-Open No. 2000-105338, Japanese Patent Application Laid-Open No. 2000-131614 and Japanese Patent Application Laid-Open No. 2000-199853. All of these optical systems are comprised of prism members, so these optical systems are not enough to be light weight.
Also, Japanese Patent Application Laid-Open No. 2000-206446, Japanese Patent Application Laid-Open No. 2000-221440, etc. disclose optical systems in which a prism is used as a relay optical system and a mirror is used as an eyepiece optical system. In Japanese Patent Application Laid-Open No. 2000-206446, the eyepiece optical system is comprised of a half mirror and a concave mirror and therefore, the thinning of the optical system has been difficult and the loss of quantity of light has tended to become great. Also, in Japanese Patent Application Laid-Open No. 2000-221440, the eyepiece optical system is comprised of an eccentric mirror and therefore, the thinning of the entire system is difficult.